Starting with tomorrow's trunk build of Firefox, it'll be possible to enable auto-offline support in the browser. Right now, it only works on Windows (see bug 76111 for details), but support for other platforms is coming soon.
To enable this feature in a Windows build, you unfortunately need to disable auto-dialer support. In the future, this won't be necessary as we'll be able to support both auto-offline and the auto-dialer. For now, however, you will need to disable the "network.autodial-helper.enabled" using about:config.
The plan is to get this feature in for Bon Echo Alpha 2 for more widespread testing.